A key step in the fabrication of semiconductor wafers is the patterning of wafer layers using photolithography. In brief, photolithography involves the selective exposure of the wafer being fabricated to light having particular frequency and energy characteristics. The wafer usually will have been pre-treated with so-called “photo-resist” or other suitable chemical. When exposed to light, the photo-resist undergoes a chemical change that modifies its ability to be removed from the wafer. In order to form patterns in wafer layers, photo-resist is selectively exposed to light by means of an optical mask bearing the desired pattern. The exposed photo-resist can then be selectively removed in order to generate a pattern in the wafer having features that, in the ideal case, exactly duplicate those of the optical mask. Subsequent etch or deposition of the wafer can then be performed in those areas where the photo-resist has been removed.
The extreme miniaturization of features in semiconductor devices places an absolute premium on the accuracy of the steps involved in their fabrication, as well as on freedom from even small errors in each step. These errors may tend to arise as a result of a number of factors and are difficult to prevent. Contaminations during the fabrication of photomasks, for example, as well as embedded impurities in substrates, resists, or other involved materials, may give rise to defects in the mask after the patterning process. As a result, the repair of defects in photolithography masks necessarily turns out to be a regular phase of the manufacturing process.
A photolithography mask typically has comparatively transparent portions, through which light is intended to pass to the wafer being patterned, and comparatively opaque portions, through which no light is intended to pass. Mask defects therefore fall into two categories: “clear defects,” in which a clear or transparent area has been created in a region on the mask that was supposed to remain opaque; and “opaque defects,” in which an area intended to be transparent has instead been caused to become comparatively opaque to the transmission of light. Because photomasks can be expensive to create and are expected to be used repeatedly, yet may be subject during their use to insults leading to the creation of defects that would otherwise render them unusable, it can be far more economical to repair the masks than be forced to replace them.
Indeed, defects arise as a result of the very manufacture of the mask itself. Since the mask is a “negative” or an original, a zero-tolerance is applied against defects of a certain size. Repair is a fixed part of the standard mask-making flow.
Most photolithography masks employ two primary layers: (1) a transparent. e.g., glass, substrate layer having high light transmittance; and (2) a masking, opaque layer having a lower transmittance. Depending upon the type of photolithographic mask, of which there are several, the transmittance of the opaque layer may actually be other than non-zero. In certain attenuated phase shift masks, for example, the opaque attenuator layer may have a transmittance of approximately 6%. Other phase shift masks may have different transmission, such as a high transmittance of 17%, for one example.
Not only is the transmittance of each of the mask layers important, but so is the index of refraction, which imparts a phase-shift to any light passing through the material. In an attenuated phase shift mask, for example, the opaque layer is selected to have such a thickness and index of refraction that it brings about a 180° phase angle of the light it transmits relative to light that passes through the glass substrate layer alone. In some attenuated phase shift masks, molybdenum silicide is used to form the attenuator.
As the size of features of integrated circuit devices continues to shrink, a premium is placed on the ability to resolve a projected light image onto a photo-resist layer to the greatest extent possible. The phase angle of light passing through the substrate and the attenuator, as opposed to the substrate alone, therefore has great practical importance. In an attenuated phase shift mask, for example, a 180° degree phase angle between the attenuated and unattenuated light leads to a maximal contrast—and therefore resolution—of the light on the photo-resist. This is because, at the interface between the attenuator and non-attenuator regions of the mask where interference is apt to occur, the 180° phase angle leads such interfering light waves to destructively interfere and thereby cancel one another. This approach, and others, thereby make it possible to create crisply defined features on the photo-resist material and, by extension, on the semiconductor device itself.
Opaque defects are typically treated through the application of a focused laser or ion beam, during which the residual material evaporates or is sputtered away. In the case of clear defects, on the other hand, a material needs to be deposited. The choice of material to be used for the deposit is driven mainly by the demand for a local and accurate placement of the material out of the gas phase. For binary masks, the only requirement regarding optical performance is a transmittance that is close to zero.
In the case of attenuated phase shift masks, in which the mask material is, for example, molybdenum silicide (MoSi2) or other suitable material, the situation is different. The masking material in attenuated phase shift masks actually transmits a certain amount of light, but also introduces a phase shift, relative to the fully transparent areas, of 180 degrees.
The task of repairing a clear defect is therefore a complicated one. Depositing MoSi2 is not technically feasible. Rather, the most common repair techniques use carbon-based material, although other suitable materials, such as zirconium oxide-based materials, may also be used. As result of using a material different from the surrounding mask material, the transmission and the phase associated with a repair site generally will not match the surrounding masking material. This disparity, in turn, leads to critical dimension (CD) errors associated with the repaired feature after wafer exposure. Adjusting the thickness of the repair patch may make it possible to match at least the transmission at the repair site, but a phase error remains.
With known approaches to the repair of attenuated phase shift masks, which typically employ the deposition of a comparatively opaque carbon-based or other suitable material at the defect site, if the material is deposited with care at a particular thickness, it is possible to match the transmittance of the repair material with that of the attenuator. The differing optical properties of the repair material relative to the attenuator generally make it difficult or impossible simultaneously to match the phase of the attenuator. As a result, although the intensity of light transmitted through the repair region might be properly corrected, the phase is not, and the interference that occurs at the interface between the repair region and those regions that adjoin it will not be destructive interference. The resolution of features in the vicinity of the repair can therefore be expected to be lower than the degree of resolution expected of the photolithographic mask prior to the defect.
There is, therefore, an unmet need for a method of repairing defects in photolithographic masks, such as attenuated phase shift masks, that permits both the transmittance as well as the phase of the light passing through the repair to match those of the light passing through the area of the defect prior to the repair.